In classical electromagnetism, Ampère's circuital law (not to be confused with Ampère's force law that Ampère discovered in 1823[1]) relates the integrated magnetic field around a closed loop to the electric current passing through the loop. James Clerk Maxwell (not Ampère) derived it using hydrodynamics in his 1861 paper On Physical Lines of Force[2] and it is now one of the Maxwell equations, which form the basis of classical electromagnetism.